Conventional backpacks for hikers and others to carry loads typically utilize either a rigid tubular metal frame secured externally to a pack, or a frame having a limited degree of flexibility which is mounted internally within the pack. The frame serves the purpose of both supporting the load placed within the pack, and distributing the load to the user's hips and shoulders. External frame backpacks are typically well suited for use with heavier loads, but are often uncomfortable due to the rigidity of the frame.
Internal frame packs are designed to more closely hug and conform to the user's body, as compared to external frame packs, as is often desirable for stability for off-trail activities. Conventional internal frame packs include a padded back panel which rests against the wearer's back. A pocket is formed within the pack on the opposite side of the padded back panel, which receives a semi-rigid planar support sheet. The support sheet is typically formed from a sheet of plastic material or lightweight metal, such as aluminum. The support sheet may be contoured, sometimes on a customized basis, to follow the contour of the back and spine. The resulting frame formed by the pad and support sheet is typically more flexible than an external frame, because of the limited flexibility of the planar support sheet, the resilient cushioning of the back panel pad, and the typically less extensive vertical and horizontal span of the frame as compared to that of an external frame. Internal frame backpacks are typically more flexible and conform to the wearer's back to stabilize the load during active movement, but may not be as effective at distributing the load to the hips and shoulders as an external frame pack. Conventional internal frame packs may also undesirably concentrate the load in part on the lumbar region of the back.
A further limitation of conventional backpack frames is that differing loads require differing levels of support. A heavy load typically will require a very stiff frame, such as may be provided by external frame packs, or by some internal frame packs which have only a very limited degree of flexibility. A smaller load, however, may be well supported by a more flexible internal frame pack which is more comfortable to wear. Thus, an individual who at times carries loads of differing magnitude, such as for differing duration hikes, would optimally need at least two packs designed to meet the varying load conditions.
A still further limitation of conventional pack designs, particularly those which have a higher degree of stiffness and rigidity to carry heavier loads, is the inability of the pack to flex sufficiently to conform to a user's back as the user bends and twists during hiking. This results in the pack at times bearing uncomfortably on small portions of the back during bending and twisting movements, even in packs where the frame has been contoured to follow the profile back in the normal upright position.